The research proposed here is a continuation of studies aimed at understanding the way in which synapases between mammalian neurons are established and maintained. In particular, we want to know how synapses are made in correct numbers between appropriate types of nerve cells, and the mechanism of their maintenance throughout life. These aims will be pursued in the peripheral sympathetic system of guinea-pigs using electrophysiological techniques, electron microscopy, and a variety of histological methods. The sympathetic system has been chosen because a) The ganglion cells are accessible, b) Much of their function and structure is already known, c) Connections in sympathetic ganglia are likely to obey the same rules as synapses in other parts of the mammalian nervous system, and d) Some of the rules that govern synaptic connectivity in sympathetic ganglia have already been established. The research will be carried out along several related lines: 1. The nature and basis of the selective synaptic connections evident in mature sympathetic ganglia. 2. The development of synapses, particularly the relation of synapse elimination to selective synapse formation. 3. The role of ganglionic targets in determining the type of innervation ganglion cells receive. 4. The intrinsic properties of ganglion cells as determinants of the innervation they receive. 5. The specificity of postganglionic reinnervation. 6. The effects of aging on connections in sympathetic ganglia. The significance of this work lies in understanding how neuronal synaptic connections are normally controlled.